When all the bars are full, sparkles appear around the Sim, and he/she receives a 1.5 times bonus for all actions. You can fill the bars by doing specific tasks. If the bars are red, the Sim receives less Simoleons and XP from actions. Also, they walk slower, hunched, and yellow fume radiates from them. If the rating is exceptionally low, the Sim will refuse to go to work and not visit another Sim's house.